1. Field of the Invention
The invention concerns a device for measuring a subject's ocular parameters, in particular the interpupilary distance, for the purposes of fitting a pair of eyeglasses.
2. Description of the prior art
When fitting a person with a pair of eyeglasses the lenses must be fitted in the frame so that their principal optical axes coincide with the optical axes of the corresponding eyes under average conditions of use. The frame acts as a secondary positional reference for the lenses, its position relative to the face of the person being essentially defined by the nose pads bearing on the opposite sides of the bridge of the nose and secondarily by the side pieces bearing on the ears.
The position of the lenses relative to the frame is conveniently defined in cartesian axes with a horizontal (relative to the face of the person) axis of abscissae and a vertical axis of ordinates, through the abscissa and the ordinate of the optical center of the lens. The origin of abscissae is naturally situated in the vertical plane of symmetry of the frame. For reasons of convenience in finishing the lenses, there is generally selected as the origin of the ordinates the horizontal straight line which passes through the bottom part of the frame hoops so that the ordinate is the distance between the optical center of the lens and its bottom edge.
The reader is reminded that the interpupillary distance is the distance between the optical axes of the eyes when focussed at infinity. The (vector) difference between the abscissae of the optical centers of the lenses must correspond to the interpupillary distance of the person.
French Pat. No. 1 506 352, filed Aug. 4 1966, describes an improved device for measuring the interpupillary distance by determining for each eye the position of the reflection on the cornea of a luminous point situated at infinity, with the gaze focussed at infinity. Analogous determinations are made with the gaze focussed on a point at a finite distance, in which case the optical axes of the eyes converge.
The device of the aforementioned patent essentially comprises a box in which there are disposed a lens which can be moved parallel to itself along its principal optical axis, a light source at the focus of the lens when the latter is at an origin position farthest removed from the source, this being offset geometrically from the optical axis by means of a semi-reflecting mirror disposed at 45.degree., and on two opposite sides of the box, perpendicular to the optical axis of the lens, an eyepiece situated on the optical axis at the focus of the lens in the origin position and a front plate comprising nose pads and two openings to simulate an eyeglass frame. The openings surround the intended location of the lenses and are provided with mobile markers forming graticules. For taking the measurements the mobile markers are made to coincide with the reflections of the point source on the corneas, as seen through the eyepiece. It will be noted that the position of the lens defines a virtual distance from the point source for the subject whereas the point from which the operator looks through the eyepiece is continuously coincident optically with the point source. Measuring the interpupillary distance for a gaze convergent to a near virtual point does not introduce any angular parallax error and the position of the graticule in the plane of the windows, which corresponds to the general plane of the lenses, defines the required position of the optical center of the lenses.
while the optical design of this device is excellent, the device suffers from imperfections in respect of the mechanical part controlling the displacement of the graticules. The necessary precision of a few tenths of a millimeter imposes the use of micrometer screws or racks for displacing the indexes, resulting in devices which are fragile and slow to operate. The position readings are difficult, involving verniers or drums associated with linear scales as soon as the precision required exceeds one half-millimeter. Furthermore, displacement of the graticules in two mutually perpendicular directions seriously complicates the mechanical control and display arrangements. In fact, the device as described in the aforementioned patent comprises only horizontally displaceable indexes, for determining principally the interpupillary distance and secondarily the distance of the eyeglass frame from the eye, by viewing in profile.
An object of the invention is a device for measuring ocular parameters capable of quickly producing precise data for matching lenses in an eyeglass frame to the ocular parameters of a person.
Another object of the invention is a device of this kind which provides, in addition to the interpupillary distances between the pupils and between each pupil and the axis of symmetry of the frame, the distance between the pupil and the lower edge of the frame, and which materializes the position of the lenses in the frame.
A further object of the invention is a device of this kind which produces directly usable numerical data.